Miles Adler-Hart discovers an addendum to the old adage that eavesdroppers never hear any good of themselves. In his case, the Santa Monica boy never hears anything good, period, in Mona Simpson’s poignant sixth novel, Casebook.

“I was a snoop, but a peculiar kind. I only discovered what I most didn’t want to know,” Miles tells readers.

Afraid that his parents are considering divorce, he and his partner-in-crime, Hector, use a walkie talkie hidden under his parents’ bed and an old phone with the mouthpiece stopped up with cotton balls, Silly Putty, and nail polish to eavesdrop on Miles’s mom, a professor who calls herself “pretty for a mathematician.”

Eavesdropping is immediately painful – “the words pressed on me, like sharp cookie cutters” – but his childhood feels so shaky that Miles can’t make himself stop. Both boys adore The Mims, as Miles calls her – with Hector renaming her Irene Adler, known to Sherlock Holmes as The Woman, in the book. (“Casebook” also gets its title from the legendary consulting detective, and Miles receives the complete stories as a gift.)

“Does everyone want his mother honored?” Miles wonders when one of his twin sisters complains about having a nerd for a mother. “But I valued her. I wished I could take what I knew was inside her and show it around, like a mineral you could bring to class.”

“Casebook” is written as a novel within a novel. It’s ostensibly a prequel to a cult comic book, “Two Sleuths,” written by Hector and Miles and published by Neverland Comics, their local hang-out. The snooping starts when Miles is nine and continues after high school, long after his fears have become a reality.

Simpson includes excerpts from the boys' comic book and the occasional footnote from Hector, but these are too sporadic to be entirely effective. At this point, of course, the unhappy hollowness lurking behind the manicured lawns of upper-middle-class homes has become such a literary staple that one can imagine Oliver Twist patting private school pupils on the back on their way to violin lessons, murmuring a sympathetic “There, there.”

But Simpson (“Anywhere But Here”) is more accomplished than most at suburban angst. And the boys’ adventure tale – complete with a mystery, walkie talkies, and a treehouse – ultimately is an extended love letter by a son about his mother.

“We were different from other families. My dad had chosen to be. The Mims just was. She couldn’t help it. She probably would rather have been more like everyone else,” Miles writes. After his parents divorce, his mom starts dating Eli Lee, who works for the National Science Foundation and holds out the promise of happiness, or at least security, for both Miles, his mom and his twin sisters, whom Miles nicknames The Boops.

Happiness, and its elusiveness, is a running theme throughout the novel. Miles tries to define it, as if, by pinning it down, he might be able to secure more of it for his mom. “‘Hope for happiness is happiness,’” he tells Hector, and later thinks, “Happiness really may be just a form of relief.”

One of the most poignant scenes comes early on, in the kitchen of a friend. “My mother never seemed happier than on that day, eating chocolate pudding in the cold. A mother’s happiness: something you recognize and then forget; it didn’t seem to matter much at the time, though it spread through our bodies. How did I know a moment like that was something I’d collect and later touch for consolation?” Miles writes.

As time goes on, the boys start uncovering discrepancies in Eli’s story. Realizing they’re in over their heads, they enlist the services of a real private investigator and embark on an inventive course of revenge.

Simpson is equally adept at capturing the world of moneyed life in a California beach town once there is less of it and the uncertainty of children navigating the new rules of divorce. “[T]here’d been lots of asking how we felt. Not that how we felt made any difference,” Miles remarks. “We had what we had before but less of it. And we never knew when it would end.”